class Solution {
public:
    int maxProfit(vector<int>& prices) {
      int len = prices.size();
      if(len == 0||len == 1) return 0;
      vector<vector<int> >dp(len,vector<int>(4,0));
      dp[0][0] = -prices[0];
      for(int i = 1;i < len;i++){
        dp[i][0] = max(dp[i-1][0],max(dp[i-1][3] - prices[i],dp[i-1][1] - prices[i]));
        dp[i][1] = max(dp[i-1][1],dp[i-1][3]);
        dp[i][2] = dp[i-1][0] + prices[i];
        dp[i][3] = dp[i-1][2];
      }
      return max(dp[len-1][1],max(dp[len-1][2],dp[len-1][3]));
    }
};
